In recent years, it has become apparent that conventional methods of generating electricity will soon be insufficient to meet the world's ever-growing energy requirements for electric power. Several factors, including the pollution which results from the combustion of fossil fuels, the dangers associated with the operation of nuclear reactors, and the limitations inherent in the traditional hydroelectric and more modern approaches to the generation of electricity such as solar energy conversion and fuel cells have encouraged the development of alternative sources of electric power.
One of these is the use of high efficiency wind turbines as prime movers for electrical generators.
Numerous systems for generating electricity from the wind have been proposed. Among the most successful of these systems have been the horizontal axis wind turbines (HAWT's) which derive their name from the fact that the turbine rotor has a generally horizontal axis of rotation and either two or three blades. The rotor is designed to rotate under the influence of the wind. This rotation is subsequently used to generate electric power by means of a drive train, gearbox, and generator.
In a typical installation, the resulting power is supplied to the local utility by means of transformers and substation-controlled connections. Generally, wind energy projects have large numbers of wind turbine generating systems at locations with favorable wind conditions. Several of these so-called "wind farms" are located in the state of California.
In this general class of HAWT wind turbines are two notable sub-classes--stall-regulated rotors and those with active or partially active adjustable pitch rotors. Stall-regulated rotors have blades which do not change pitch and depend on careful aerodynamic blade design for peak power control. Wind turbines with active and partially active pitch systems include a mechanism, usually hydraulic, to physically change the orientation or "pitch" of the blade, either at its root or an outboard station, in such a way so as to limit the power generated by the rotor to a selected maximum value. This limitation is generally dictated by maximum power or load capabilities of other system components.
Wind turbine rotors with adjustable pitch blades are complicated and expensive because of the mechanism required to adjust the blade pitch and to control this adjustment as wind conditions change. Furthermore, maintenance of these rotors is a burden, especially in those not uncommon applications in which wind turbines are employed in considerable numbers at remote locations and in third world countries and other locales where skilled mechanics are more than likely not available.
Because of the foregoing and other drawbacks of adjustable pitch wind turbine rotors, considerable attention has been devoted to those of the self-regulating, stall-regulated type. The efficiency with which typical stall-regulated wind turbine rotors convert wind energy into useable mechanical shaft power is dominated by the aerodynamic characteristics of the airfoil sections employed in the rotor blades as well as the basic planform and geometry of the blades. Due to the large spanwise variations in the velocity and direction of the air relative to the blade, stall-regulated wind turbine blades have some unique requirements associated with efficient operation. These needs are not met by the airfoil sections developed over the past half century for use in fixed and rotary wing aircraft.
One major problem appurtenant to contemporary stall-regulated blades is that effective stall regulation cannot be achieved at peak (high wind speed) power levels without compromising low wind speed performance if a single family of airfoil profiles is employed as has perhaps most often heretofore been the case.
Another major problem with contemporary blades is the loss of rotor efficiency caused by soiling and the accumulation of debris on the leading edges of the blades. This increased surface roughness causes significant reductions in rotor efficiency.
In sum, currently available stall-regulated wind turbines have a number of deficiencies, and there is a continuing need for improved machines of this character.